Turbine problems and equipment failures are primary causes of power plant outages in the power generation industry and can necessitate costly maintenance and repair procedures. Many types of turbines are used in the generation of electrical power. Typically, a turbine rotor or wheel includes multiple rings or rows of turbine blades in spaced relationship along the axis of rotation of the turbine wheel. Most of such rings of turbine blades are structurally stabilized by inclusion of a circumferential structural band, or shroud, interconnecting all or a group of the blades at the outer tips of the blades.
The tenon is an extension of a turbine blade which is shaped to fit into an opening in the shroud. Each tenon initially protrudes through its respective opening in the shroud and the protruding end is mechanically deformed by being peened over, much like a rivet, by either manual or automatic action in order to hold the shroud firmly and securely in its assembled position, thus stabilizing the ends of the turbine blades. Each ring of turbine blades may typically include 120 to 150 individual blades and the use of a variety of methods of tenon peening creates a variety of different forms of tenon surfaces The tenon necessarily has a relatively small cross section and the physical deformation involved in the peening process results in the tenon in situ in the shroud having a generally rounded irregular surface accessible above the shroud opening through which the end of the tenon protrudes.
In operation, high rotation speeds, such as 1,800 or 3,600 revolutions per minute, vibration and temperature changes may combine to produce significant stresses in the turbine wheels and blades, and particularly in the blade tenons affixed to the shrouds. Cracks, fractures and failures of turbine wheels and blades, and particularly turbine blade tenons, are some of the most common problems encountered in turbine operation. One failure sequence is as follows. One tenon may initially have or develop a very small crack which subsequently, under operating stresses, may completely fracture. This increases the stresses on the tenons of adjacent blades, which may then also fracture, further increasing stresses on additional tenons. The segment of shroud will then have nothing to hold it in position and once a length of shroud has broken loose at high rotational speed very significant internal damage can be done to a turbine. Resulting unscheduled outages can range from days to months while repairs are completed, with major repair and outage costs to an electric utility.
Tenon failure can be avoided by timely inspection of each tenon of each ring of blades of a turbine to provide early detection of faults or cracks while they are still too small to affect structural integrity. A serious impediment to effective inspection is that the most likely location for an initial fault is in the leading edge of a turbine blade tenon at the point where it is fixed to the inside surface of the circumferential shroud, a point which is highly inaccessible in the assembled turbine wheel. Inspection by disassembly is costly and time consuming, and actually physically destructive of the tenon, since the peened-over portion must be removed thereby limiting the reusability of the turbine blade. Prior known approaches to use of ultrasonic inspection, in order to avoid the need for disassembly, have proven impractical and unreliable largely because of the small size and curved or irregular surface configuration of the peened tenon end, which generally represents the only usable and accessible external surface available for introduction of ultrasonic energy.
Typically, previously-available ultrasonic inspection systems have relied upon a wedge or block of plastic or other flexible material in contact with the tenon end in order to couple ultrasonic energy from the transducer into the interior of the tenon and allow reflected echoes to return to the transducer, to permit internal inspection. In the case of turbine blade tenons, the small non-planar surface available for such purposes makes adequate contact for efficient coupling very difficult or impossible. In addition, since unfocused beams have been used, the cross-sectional area of the ultrasonic beam may be about as large as the available tenon surface and an uneven curved or irregularly faceted tenon surface can cause severe aberration of the ultrasonic beam. Such aberrations may make it difficult to determine the actual direction or directions in which the beam is actually entering and traveling within the tenon. Also, the beam may actually enter the tenon in different modes (i.e., different ones of a number of possible shear and compression modes), so as to make accurate interpretation of any resulting data very difficult. Even if sufficient coupling could be achieved, a relatively broad, unfocused beam can illuminate a variety of features within a tenon and thereby give rise to a variety of echo returns, some of which may be larger than, and sufficient to mask, an echo from a minute flaw which must be detected.
Other difficulties in tenon inspection relate to the extended time and effort required to inspect the tenons of a turbine wheel including several rings of turbine blades, with 120 or more blades in each ring. Manual approaches are subject to operator fatigue and loss of attention as hundreds of tenons are subjected to inspection, as well as to non-uniformity of alignment of a test fixture to successive tenons.
Once the nature of the difficulties involved in achieving reliable and repeatable ultrasonic data when inspecting irregularly shaped tenon surfaces is understood, it will be appreciated that similar difficulties hamper effective ultrasonic inspection of other types of turbines and rotating structures, as well as other forms of devices where the usefulness of prior ultrasonic approaches has been seriously limited by small, curved, faceted, irregular or other shapes of access surfaces which lack a sizable flat surface in a readily usable position.
It is therefore an object of this invention to provide improved systems and methods for ultrasonic inspection and particularly such systems and methods applicable to inspection of turbine blade tenons which avoid shortcomings of prior approaches.
It is a further object to provide improved systems and methods permitting inspection via small and/or irregular surfaces by use of electronically focused ultrasonic beams coupled from a transducer array into the interior of a component while both are immersed in a fluid beam coupling medium.
Additional objects are to provide automated ultrasonic inspection systems and methods capable of automatic inspection at a plurality of points on a rotatable structure, and such systems and methods capable of automatically identifying fault conditions.